Multiplexer channel units



Feb. 18, 1969 G. L.VMATTHAEI 3,

MULTIPLEXER CHANNEL UNITS Filed May 26, 1966 INVENTOR, GEORGE L. MATT HAE I BY m 4. M Mam M M W ATTORNEYS- United States Patent 3,428,918MULTIPLEXER CHANNEL UNITS George L. Matthaei, Santa Barbara, Calif.,assrgnor, by mesne assignments, to the United States of America asrepresented by the Secretary of the Army Filed May26, 1966, Ser. No.553,604

US. Cl. 333-6 Int. Cl. H01p /12 1 Claim ABSTRACT OF THE DISCLOSURE Thisinvention relates to multiplexer channel units, and more particularly toa filter unit for separating out bands of microwave frequencies from atransmission line carrying a large range of microwave frequencies.

Microwave multiplexers are required to take many signals of differentfrequencies which are propagating on a single transmission line andseparate them out onto a number of transmission lines according to theirfrequencies. In principal this can be done by using a bandpass filter toseparate out each of the desired frequencies. If the band-pass filtersfor the various frequencies are connected in parallel, then the manyfrequencies on the main transmission line are separated out into theindividual filters. In practice, this is very difiicult to do,particularly at microwave frequencies because conventional band-passfilters will interact with each other. Interaction of the filters has anadverse effect on the performance of each filter. The undesirableinteraction effects can be reduced somewhat by connecting all thefilters to a common point. Unfortunately, when waveguides or other formsof microwave transmission lines are used, only a very limited number offilters can be joined together at one point.

Directional filters have been used with some success to multiplexerunits. These filters use a main transmission line and the signals to beseparated out are taken away by a directional filter structure whichcondutcs them out to a side transmission line. The main transmissionline has the property of always presenting a matched impedance. For thisreason, in theory any number of directional filter channel units can becascaded, without undesirable interaction effects.

Although in concept directional filters appear to be ideal asmultiplexer filters, they have some practical disadvantages. In order toobtain directional 'filter action, each resonator of the filter mustsupport two orthogonal resonant modes at the same time. Because of this,directional filters are quite difficult to tune if there are more thanone or two resonators in the filter, and usually more than two arenecessary. Also, directional filters require a relatively tight couplingto the main transmission line. This tight coupling causes a very sizablevoltage standing Wave ratio (VSWR) at frequencies off of the pass-bandof the directional filter, particularly in the case of Waveguidedirectional filters. Thus, while in theory directional filter channelunits should present a perfect impedance match at all frequencies, inpractice the mismatch introduced by practical filter units may be quitelarge.

I have invented a multiplexer channel unit that has all the advantagesof directional filter units, with considerable fewer of theirdisadvantages. Basically, my multiplexer channel unit comprises apass-band filter and a stop-band filter. The unit is designed to selecta single frequency or a band of frequencies from a transmission linecarrying a large number of frequencies. Any number of my channel unitscan be cascaded together so that all frequencies carried by a giventransmission line can be selectively taken from the line. There isessentially no interaction between the units and not tuning problemexists when my channel units are cascaded.

Therefore, an object of my invention is to provide a multiplexer channelunit.

Another object of my invention is to provide a system for selecting byfrequency one or more signals from a transmission line carrying a largenumber of signals of different frequencies.

A further object of my invention is to provide a multiplexer channelunit so designed that any number of these units can be cascaded togetherwithout any adverse effects.

The above mentioned and other objects of the invention will becomeapparent from the following detailed description and accompanyingdrawing in which:

FIG. 1 shows a strip line version of my invention;

FIG. 2 shows a waveguide version of my invention; and

FIGS. 3(a)3 (0) show in detail the tuning screw and the resonant irisconstruction of the stop-band resonators of FIG. 2.

Referring to FIG. 1, input signals are applied to a transmission line 4at the input terminal 1. Assume that a large number of signals havingfrequencies of f to f are so applied to transmission line 4 and that thesignals travel in the direction of the arrow at input 1. It will also beassumed in the following discussion that all the signals except thosehaving a frequency of f are to be present at the output terminal 2. Theelements 5, 6, 7 and 8 are resonators which are a quarter wavelengthlong at frequency h, in this case, and which are short-circuited at oneend and open-circuited at the other end. Elements 5, 6, 7 and 8 form aninterdigital band-pass filter so designed that only those signals ontransmission line 4 having a frequency i, will appear at the outputterminal 3.

The elements 9, 10 and 11 are also resonators that are open-circuited atone end and short-circuited at the other end. These elements form aband-stop filter. The band-stop filter rejects all those signals havinga frequency of f While permitting all the other signals to pass tooutput The combined action of the band-pass filter and bandstop filteris such that output 2 is effectively isolated from all those signalshaving a frequency of f and output terminal 3 is effectively isolatedfrom all those signals having a frequency different than f Any number ofunits constructed similar to the unit shown in FIG. 1 may be connectedtogether. The only difference between the units will be the frequencycharacteristics of the filters. Each unit is designed to select adifferent frequency. Of course the filters of each of the units can beso designed that each unit will select those signals falling within aparticular frequency band rather than just those signals of a givensingle frequency.

'In practice, the strip line version of my multiplexer channel unit isconstructed in accordance with conventional design techniques. By properdesign the input impedance looking in from terminal 1 can be made to benearly a perfect constant resistance. For this reason,

any number of units can be cascaded without harmful interaction effects.

FIG. 2 shows a waveguide version of my multiplexer channel unit. Thewaveguide 4 is the main transmission line. Input signals havingdifferent frequencies are simultaneously applied to waveguide 4 at theinput 1. The signals travel along waveguide 4 in the directionind-icated by the arrow. The elements numbered 6 are conventional cavityresonator-s. Each resonator is provided with a tuning screw 9. Theelement 7 is a waveguide band-pass filter. The elements number '8 arespecially designed resonators which will be more fully described. Aresidual reactance annulling screw 5 is provided at the bottom ofwaveguide 4.

It is again assumed that the multiplexer unit shown is to select onlythose signals having a frequency of f while permitting all other signalsto pass to the output port 2, then cavity resonators 6 will be designedand tuned by means of tuning screws 9 to be resonant at frequency fBand-pass filter 7 is designed to pass only those signals of frequency fTherefore, only f signals will appear at the output port 3.

Resonators 8 are designed and tuned by means of the tuning screws 10 toreject f signals while permitting all other signals to pass on to theoutput port 2. Resonators 8 form a band-stop filter. The band-stopfilter assures that no f signals will appear at port 2. Therefore, thecombined action of cavity resonators 6 and band pass filter 7 and theband-stop filter assures that only f signals will appear at a port 3while all other signals will pass undisturbed on to port 2.

Any number of waveguide units of the type shown in FIG. 2 may becascaded with little or no adverse effects. Each unit, of course, isdesigned to select a difi erent frequency or band of frequencies. Theresonators are readily brought to resonance by adjusting their tuningscrews. Tuning is a simple matter when compared with the difiicultiesencountered in attempting to tune waveguide directional filter.

As was mentioned above, resonators 8 are specially designed devices. Allthe other components are conventional waveguide components. I attemptedto use conventional iris-coupled band-stop resonators for the bandstopfilter, but I found that because of the inductive-iris couplings ofthese resonators, the structure had an excessive voltage standing ratioat frequencies off of the channel separation frequency. For this reasonI designed a novel type of band-stop filter resonator. This resonator isshown in detail in FIGS. 3 (a)-3 (c).

As shown in FIGS. 3(a) and 3*( b), a small resonant iris 13 is cut outof a slab of metal 12. The whole slab of metal with the iris is solderedinto the top wall of waveguide 4. The back wall 14 behind the iris is0.100 inch away from iris 13 while the metal wall that iris 13 is cutout of is 0.050 inch thick. Iris 13 is brought to resonance at thedesired frequency by tuning screw 10. Screw 10 is a chrome plate brassscrew which is capped by a section of dielectric material 1'5 and an endcap 16 of aluminum foil 16. Thus, the actual tuning is achieved by thedielectric portion of the screw with the aluminum foil tip. The mainadvantage of this type of resonator is that it is small and compact andoperates much like a lumped-element resonator. The use of my novelresonator resulted in a much lower off-resonance voltage standing ratiothan which occurred when I used conventional iris-coupled band-stopresonators.

The conventional iris coupled cavity resonators used for resonators 6also tended to give a sizable voltage standing ratio olf of thepass-band of the band-pass filter. This problem is solved by the use ofresidual reactance-annulling screw 5.

From the foregoing remarks it is apparent that I have invented amicrowave frequency multiplexer channel unit that has all the advantagesof the conventional prior art type of multiplexer channel units withoutthe major disadvantages of the prior art devices. Any number of mymultiplexer channel units can be cascaded without any adverseinteraction or tuning problems. Furthermore, waveguide or strip linetechniques can be used to con struct my multiplexer channel unit.

The invention has been described with reference to two preferredembodiments. It will be obvious to those skilled in the art that variousmodifications and changes can be made to the embodiments shown anddescribed without departing from the spirit and scope of the inventionas defined in the appended claim.

I claim:

1. A multiplexer channel unit comprising a waveguide transmission line;a waveguide band-pass filter coupled to said transmission line, saidband-pass filter having first, second and third tunable resonatorsconnected in series and a waveguide filter section connected to saidthird resonator; and a band-stop filter coupled to said transmissionline, said band-stop filter having first and second iriscoupledresonators, each of said iris-coupled resonators having a slab of metalwith an iris cut out of said slab, a back plate mounted in back of saidiris plate in such a manner that said iris is .100 inch away from saidback plate and a chrome plated tuning screw having a dielectric cap andan end cap of aluminum foil.

References Cited UNITED STATES PATENTS 1,469,832 10/19-23 Hamilton 333-6XR 2,432,093 12/1947 "Fox 333-7'3 2,588,226 3/1952 Fox 333-73 2,816,27012/ 1957 Lewis 333-73 XR 3,292,075 12/1966 Wenzel. 3,327,255 '6/ 1967Bolljahn et al 33373 3,345,589 10/1967 Di Piazza 333-73 3,348,17310/1967 Matthaei et al. 333-73 FOREIGN PATENTS 683,068 4/1964 Canada.

HERMAN KARL SA'ALBACH, Primary Examiner. M. NUSSBAUM, AssistantExaminer.

US. Cl. X.R.

